Not Applicable
Not Applicable
1. Field of Invention
The present invention relates to mechanical actuators. More specifically, the present invention relates to artificial muscle analogs which do not rely on electromagnetism for their motive force. Certain aspects of the present invention relate to robust, low-cost exoskeletal limbs powered by such muscle analogs, which have applications in many fields including robotics or prosthetics. A final aspect of the present invention relates to a particularly simple means of controlling such exoskeletal limbs, using a novel electro-pneumatic feedback loop.
2. Description of Prior Art
Historically, machines have been invented using an almost endless array of different mechanical methods to induce physical movement.
In recent years, however, most such machines have been based on some form of electromagnetic force, especially electrical motors. In some cases, this has been due more to the overwhelming prevalence of electricity and especially the electrical motor as a motive device, rather than the inherent superiority of this approach for the particular problem. Although an electrical motor is arguably the best solution for many problems, especially those requiring rotary motion, other situations exist where alternate approaches have inherent advantages. Yet relatively little research has been done into other motive methods, due in part to the tremendous popularity of the electrical motor.
One situation where an electromagnetic actuator is not particularly ideal is when a relatively-slow, controlled but powerful linear actuation is desired, especially when the desired machine must copy biological motion. For example, consider what is needed to build a mechanical limb that mimics a human arm. First limiting consideration only to the major biceps and triceps muscles used for flexion and extension of the arm, respectively, one requires a pair of xe2x80x9cmuscle analogsxe2x80x9d or artificial muscles which can each contract with hundreds or thousands of kilograms of force, yet which weigh only a few kilograms. The muscle analogs should each be capable of pulling a xe2x80x9ctendon analogxe2x80x9d or cable through several centimeters of travel. Ideally, the muscle analogs should be adjustable in their xe2x80x9cpullxe2x80x9d throughout the range of motion. A feedback mechanism should be available so that the mechanical limb can be held at any position throughout the range of motion. And to best mimic biological muscles, the available pulling force on the cable should be greatest when the muscle analog is extended, falling as the muscle contracts; this tendency offsets the reduced mechanical advantage inherent in biological jointed limbs at extension.
The characteristics of a typical electrical motor are not well suited for such an application. For a variety of reasons, a power-conversion assembly is generally required to convert the output power of the motor to a usable form. First, the rotary motion is not desired, and must be converted to a linear motion through the addition of further hardware such as gears or threaded shafts. Also, a typical motor has optimal performance at a higher RPM than is desirable for such an application, so further speed-reducing gearing is also required. And since a powerful actuation is desired, the gearing used to reduce the speed and linearize the motion must efficiently take advantage of the mechanical advantage involved in such a speed reduction. Finally, an electrical motor does not exert force unless it is drawing power; so if it is desired to maintain an actuator in a particular position once it is moved there, some further hardware in the form of a braking means is also required. And the weight of this overcomplicated solution, including the electrical motor, the associated power-conversion assembly, and braking means, can be substantial. Clearly, there are inherent disadvantages in using electrical motors for such applications.
Specialized motors, such as stepper motors, are sometimes used for such applications, since they can generally run at slower speeds, closer to the desired speed of such actuators. But these are even more complicated and expensive to make than standard motors. And stepper motors are generally a less powerful form of electric motor; so if the stepper motor is run at a slow speed without speed-reduction/force-increasing gearing, a motor large enough to develop sufficient force will generally be even heavier than a more-typical motor with its associated gearing.
Solenoids would seem to be a better fit than motors for use as such a linear actuator, since they are inherently linear in function. A solenoid is an electromagnetic coil which pulls a ferrous core or piston into the coil""s center when current is allowed to flow. However, solenoids are also relatively heavy and inefficient for a given strength, and again they require power to hold their position. And solenoids tend to be xe2x80x9cdigitalxe2x80x9d in nature, since the pull of the solenoid actually increases as the piston is pulled in. The maximum strength of the pull is at the wrong end of the motion, resulting in biologically-unnatural jerky motions which tend to accelerate toward an abrupt stop. And, using a typical solenoid, there is no good way to stop the motion partway, or lock the solenoid in a partially-closed position.
With these inherent problems using electromagnetic force, alternate approaches have been investigated and indeed are used today in certain instances of linear actuation.
For example, one novel approach uses xe2x80x9cmemory-wirexe2x80x9d which changes length and/or shape when a current is passed through it. Such muscle analogs are extremely simple, light and small. However, this technology is still primarily a lab curiosity, since the available force is very small and the power efficiency is quite low.
Other non-electromagnetic approaches in the prior art have been more successful. For example, hydraulic cylinders enjoy a dominant role where the linear actuation desired must be particularly powerful. A hydraulic cylinder is a piston/cylinder arrangement wherein a piston is forced out when a essentially-incompressible pressurized fluid is allowed to enter a cylinder chamber when a control valve is opened. Double acting hydraulic cylinders are also available, wherein two opposing chambers are present so that the piston can be forced out or pulled in, in order to achieve the desired motion, a user opens the appropriate control valves to pressurize one chamber while simultaneously venting the opposing chamber to an unpressurized fluid reservoir. In a hydraulic cylinder, the available force is essentially constant throughout the range of motion, dependent only on the pressure of the fluid and the cross-sectional area of the piston. And since the fluid is incompressible, hydraulic cylinders lock the actuator solidly in position when all the appropriate valves are closed. However, hydraulic cylinders are generally quite slow and heavy, and are used only when the desired motion need not be particularly fast.
Analogously, pneumatic cylinders are sometimes used, with compressed air or other gases replacing the essentially incompressible but comparatively viscous hydraulic fluid in a hydraulic cylinder. Such pneumatic cylinders generally actuate much faster than comparable hydraulic cylinders. Double acting pneumatic cylinders are also available, able to exert force both during extension and retraction by pressurizing one chamber and venting the opposing chamber (probably to the atmosphere if compressed air is used). And, like hydraulic cylinders, the available force is essentially constant through the range of motion, dependent only on the gas pressure and the piston cross section. However, since the fluid now used is a compressed gas which is not incompressible in any sense, the actuator does not truly xe2x80x9clockxe2x80x9d in place when the valves are closed. Instead, the pneumatic cylinder acts more like a spring, with an increasing force opposing deviation from the desired actuator position. This is actually more akin to biological muscle force than a truly-locking actuator. Of course, like a spring, there may be oscillations in the actuator positioning if appropriate damping is not included in the design. Pneumatic cylinders are generally used at considerably lower pressures than hydraulic cylinders due in part to the greater danger associated with compressed gases compared to pressurized fluids. Pneumatic cylinders are thus usually considerably less powerful than an hydraulic cylinder of similar size; however, since they can therefore be built less strongly, and since their fluid is a gas rather than a liquid, pneumatic cylinders are also typically much lighter than their hydraulic counterparts.
Hydraulic and pneumatic cylinders have some limitations in common. Both require seals, machining and close tolerances in their manufacture, and are therefore relatively expensive to make. And both require cleanliness and care in their use. In particular, the mirror-smooth machined surface of the piston (which is often exposed when the actuator is extended) must not be bent, scratched or roughened if the cylinder is to function properly. Even microscopic particles introduced into the hydraulic fluid, for example, can score this surface and ruin a cylinder; so high-quality filtration of the pressurized fluid is necessary. Such limitations can cause problems for cylinders used as actuators in a dirty or abrasive environment, or where the actuator may be subjected to impacts or other abuse.
Still, of the available widely-used technologies in the prior art, pneumatic cylinders represent the closest match for use as muscle analogs. Although less force is produced than for a hydraulic cylinder of similar size, this force is yet sufficient to mimic biological strength. This has led to the investigation of other embodiments of pneumatic muscle analogs besides the simple piston/cylinder arrangement.
One rarely-seen type of pneumatic actuator which would seem to be particularly appropriate for use as a muscle analog is the class of devices collectively known as McKibben artificial muscles. These are cylindrically symmetric devices comprising an expandable, rubberlike bladder or balloon enclosed within a sheath loosely woven in diagonal patterns from relatively-unstretchable fibers which connect at the ends to tendon analogs; the sheath resembles a familiar xe2x80x9cchinese finger puzzlexe2x80x9d. When the bladder is inflated, the woven sheath is forced to expand in diameter around the bladder and therefore must contract in length, pulling on the tendon. Such a device has most of the advantages of a single-acting pneumatic cylinder, with much less weight and a less expensive manufacturing process; no metal or machining is generally required.
Like a pneumatic cylinder, the actuation can be held in place by closing a valve through which compressed fluid is introduced into the balloon. Of course, since a McKibben muscle only pulls in one direction, this will only oppose further motion in the same direction. However, even this aspect is much like a biological muscle, where a pair of opposing muscles (like the biceps and triceps in a human arm) are required to hold a lever (the forearm) in a given position. A pair of McKibben muscles, appropriately mounted, can replace a double acting pneumatic cylinder in most applications, with a considerable reduction in weight. The range of motion, force and distribution of force through the range of motion are all fairly close to the biological equivalents in an human arm; and the weight of these muscle analogs is even less than equivalent biological muscles.
Actuators like the McKibben artificial muscle have been known for decades. Unfortunately, however, there are practical problems with this technology which have limited the popularity of these devices and kept them from widespread use. First, the functionality of the McKibben muscle is dependent on quite special physical properties required of the enclosed bladder. Actuating the muscle requires the bladder to expand from its resting size to a much-larger inflated diameter; this requires the bladder material to be stretched several times its resting dimensions. Each actuation of the muscle requires a separate expansion of the bladder. Thus, the bladder material of a McKibben muscle must be stretchable without damage many thousands of times over a period of years, in order to achieve reliability equivalent to competing technologies. And although in some instances such long-term reliability has been achieved with other inventions using stretchable materials, such as rubber inner tubes, the material has generally not been required to expand and contract so many times, or so near its elastic limit. For example, an inner tube is typically inflated only a few times in a lifetime of use, and is supported within a toroidal chamber formed by its enclosed wheel and the semi-rigid tire surrounding it, once inflated. And typically, the minor radius of this toroidal chamber is not significantly larger than the minor radius of the inner tube before the rubber begins to stretch; so the rubber in an inner tube is not required to expand near its elastic limit in use. The rubber inner tube is a successful idea because the material property actually required of the rubber is reliable flexibility through generally low-amplitude deviations from a prescribed shape, centered around an operating point much below the elastic limit of the material. This property is much easier to attain than reliable flexibility through unsupported high-amplitude stretching near the elastic limits of a material.
In practice, it is difficult or impossible to find materials which are capable of reliably stretching so far, so many times. According to recent publications, the best and most reliable material known for the bladder in a McKibben muscle is still natural latex rubber; so essentially no advances in materials for this application have been made in the decades since its invention. Clearly, the McKibben muscle suffers from reliability problems brought on by its dependence on remarkable material properties which are difficult or impossible to realize. The few commercial manufacturers of McKibben muscles today are usually unwilling to provide even an estimate of the life expectancies of their products, which puts them in an unfavorable light when their products are compared with competing technologies that provide not only estimates but guarantees of reliability.
Further, despite the advantages inherent in its design, the McKibben muscle has never achieved wide acceptance, possibly due to this difficulty in producing materials which can deliver reliability even approaching the reliability of competing technologies. And even though the design is simple in concept, the materials typically used inexpensive, and the manufacturing process simple, McKibben muscles are not significantly less expensive than competing, more-reliable technologies. In fact, McKibben muscles commercially available today are comparable or greater in price than comparable pneumatic cylinders, even with their expensive and complex machined metal design.
And the McKibben muscle is somewhat difficult to work with. In some ways, it resembles a biological muscle too well. For example, it is a relatively fragile structure, even more so than a protoplasmic muscle (which does not fail if punctured with a pin); yet it lacks the self-healing mechanisms which make biological structures so reliable. Instead, it requires protection from a hostile environment. A robust protective chamber built around the structure is required if the user wants to ensure that the bladder is not punctured and that the woven cover is not frayed. Yet enough room must be allowed for the bladder to inflate in order for the artificial muscle to work correctly.
And when McKibben muscles are used to drive a mechanical limb, rigid support members to mimic the bones of the limb must be present, and are often hollow. Yet the McKibben muscle does not recognize the existence or take advantage of these coexisting structures, despite its need for protection. In short, the McKibben muscle closely mimics endoskeletal biological muscles, but its need for protection and relative unreliability have kept it from becoming significant in the marketplace.
Objects and Advantages
Accordingly, several objects and advantages of the present invention are:
1. To provide a linear actuator which mimics an organic muscle.
2. To provide a muscle analog with strength comparable or greater than that of an organic muscle of similar size.
3. To provide a muscle analog with speed comparable to that of organic muscles.
4. To provide a muscle analog capable of pulling a cable through a range of motion similar to that of an organic muscle.
5. To provide a muscle analog whose force throughout its range of motion decreases from a maximum at extension, like an organic muscle.
6. To provide a muscle analog which is lightweight and compact.
7. To provide a simple, low-cost and easily-manufacturable muscle analog.
8. To provide a muscle analog which does not rely on electric motors or other forms of electromagnetic force for its motive power.
9. To provide a muscle analog which is capable of holding at positions intermediate through its range of motion, without drawing power.
10. To provide a muscle analog which does not require materials with unusual, exotic or difficult to attain material properties.
11. To provide a muscle analog with high reliability.
12. To provide a muscle analog which synergistically uses coexisting structural elements as part of its own design.
13. To provide a simple, low-cost method of building robust, reliable exoskeletal robotic limbs.
14. To provide a simple, low-cost method of controlling exoskeletal robotic limbs.
A novel artificial muscle, designed to be enclosed within a substantially-rigid substantially hollow member, pipe or exoskeletal bone 10, advantageously chosen from among coexisting structural elements, comprises a bladder 120, an artificial tendon or cable 130, and a low-friction cable-positioning means or roller 140, affixed so as to constrain said cable to move freely longitudinally along a first side of the pipe interior. Said bladder is placed inside the pipe and affixed along said first side of the pipe interior. Cable 130 is attached along said first side of said pipe, at a point of attachment 150 placed longitudinally past the bladder. The cable passes from point of attachment 150 over bladder 120 and through low-friction cable-positioning means 140. A hose 160 is attached to bladder 120, through which the bladder may be inflated or deflated. When the bladder is deflated, the cable runs in an essentially straight line over the deflated bladder, along said first side of the pipe interior. When the bladder is subsequently inflated, it forces the portion of the cable passing over the bladder toward the opposing side of the pipe interior, which pulls cable 130 in through the low-friction cable-positioning means.